Metallocene catalysts have been used to manufacture polyolefins for many years. Countless academic and patent publications describe the use of these catalysts in olefin polymerisation. Metallocenes are now used industrially and polyethylenes and polypropylenes in particular are often produced using cyclopentadienyl based catalyst systems with different substitution patterns.
The two most important physical properties of isotactic polypropylene (iPP) are its average molecular weight and its melting point (Tm), the latter being mostly determined by the degree of stereoregularity (isotacticity) of the polypropylene chains.
The Ziegler-Natta catalyst systems known in the literature can produce iPP with high molecular weights together with moderate to high isotacticities and melting temperatures (Tm). The Tm (measured by standard DSC methods) of non-nucleated iPPs are in the range of 160 to 165° C.
In the case of metallocenes, there are very few examples which can produce iPP having both very high molecular weights and high melting points. For example rac-Et(2,4,7-Me3Ind)2ZrCl2 can produce isotactic polypropylene with a molecular weight of 1,900,000 g/mol and a melting point of 168° C.
In order to achieve such high values, a polymerization temperature of −30° C. is necessary. When the polymerization temperature is increased to 30° C., the melting point of the resulting polypropylene decreases to 158° C. A polymerization temperature of −30° C. is however, far too low for polypropylene manufacturing in commercial plants, which need to be operated above 60° C. When used at industrially useful polymerization temperatures, this same metallocene yields low molecular weight polypropylenes with relatively low melting point. For example at 70° C., rac-Et(2,4,7-Me3Ind)2ZrCl2/MAO yields a polypropylene of molecular weight of only 30,600 with a melting point of only 145° C.
In U.S. Pat. No. 7,405,261, rac-Et[2,7-Me2-4-(4-tBuPh)Ind]2ZrCl2 is reported to produce iPP with a melting point of 156° C., by polymerizing liquid propylene at 65° C.
WO2009/054831 describes zirconocenes with a 2-methyl-4,7-aryl substitution pattern, such as rac-Me2Si[2-Me-4,7-(4-tBuPh)2Ind]2ZrCl2. The melting points of the homopolymers are still quite low, being in all cases below 150° C. despite the relatively low polymerization temperature of 65° C.
WO02/02576 describes conventionally supported metallocenes such as rac-Me2Si[2-Me-4-(3,5-tBu2Ph)Ind]2ZrCl2. These metallocene catalysts, activated with MAO or a borate, on a silica support, at a polymerisation temperature of 60 or 70° C., give iPP with Tm between 156 and 159° C.
The metallocene rac-9-silafluorenyl-9,9-[2-Me-4-(3,5-tBu2Ph)Ind]2ZrCl2 also gives high melting point iPP and are described in WO02/02575.
In general however, metallocene catalysts, when used under industrially relevant polymerization conditions, produce iPP having melting points which are lower than the melting points of Ziegler Natta iPP, and even the best metallocene catalysts produce iPP with melting points of less than 160° C. In addition, few metallocene catalysts can produce iPP having both high melting point and high molecular weight at polymerisation temperatures above 60° C.
In order to overcome this inherent limitation of metallocene catalysts, and in order to produce polypropylenes having both high melting points and high molecular weights, we have developed a new family of catalysts comprising substituted bis-indenyl complexes.
Whilst the bisindenyl complexes of this invention are known, we employ these complexes in solid particulate yet unsupported form to make a new family of catalysts with interesting properties. These metallocenes have been found to produce isotactic polypropylenes with surprisingly high melting points and very high molecular weights.
The catalysts of the invention comprise a bridged bisindenyl metallocene complex with a substituted aryl group at the 4-position of an indenyl ligand and at least one non hydrogen substituent at the 2-position of the ring. The seven position is unsubstituted. Such complexes are known in the art in WO02/02576. However, the metallocene catalysts of WO02/025676, activated with MAO or a borate, are carried on a silica support. At polymerisation temperatures of 60 or 70° C. they give iPP with Tm between 156 and 159° C. but at very poor catalyst activity.
The present inventors sought a new catalyst system capable of producing, inter alia, isotactic polypropylene with high melting points, high isotacticity and high molecular weights without compromising catalyst activity at commercially relevant temperatures.